Info

Rough type (avirulent)

Smooth type (virulent)

Rough type (avirulent)

Smooth type (virulent)

Mouse dies

Heat-killed smooth type

Heat-killed smooth type plus rough type

Heat-killed smooth type

Mouse dies

Figure 1-21 The "transforming factor" discovered by Griffith was responsible for changing the phenotype of the avirulent rough type bacteria to that of the virulent smooth type.

Mouse dies

Figure 1-21 The "transforming factor" discovered by Griffith was responsible for changing the phenotype of the avirulent rough type bacteria to that of the virulent smooth type.

tially treated them with recently discovered enzymes (Fig. 1-22). Protease and ribonuclease treatment, which degraded protein and RNA, respectively, did not affect the transformation phenomenon that Griffith had demonstrated earlier. Treatment with deoxyribonuclease, which degrades DNA, however, prevented transformation. They concluded that the "transforming factor" that Griffith had first proposed was DNA. The transduction experiment of Alfred Hershey and Martha Chase also confirmed their findings that DNA carried genetic traits.

Advanced Concepts

Investigators performing early transformation studies observed the transfer of broken chromosomal DNA from one population of bacterial cells to another. Naked DNA transferred in this way, however, is a very inefficient source for transformation. Unprotected DNA is subject to physical shearing as well as chemical degradation from naturally occurring nucleases, especially on the broken ends of the DNA molecules. Natural transformations are much more efficient, because the transforming DNA is in circular form.

Cell lysate

Cell lysate

Transformation

+ proteinase

+ RNase

+ proteinase

+ RNase

Transformation

Transformation

+ DNase

+ DNase

■ Figure 1-22 Avery, MacLeod, and McCarty showed that destruction of protein or RNA in the cell lysate did not affect the transforming factor. Only destruction of DNA prevented transformation.

Plasmids

DNA helices can assume both linear and circular forms. Most bacterial chromosomes are in circular form. Chromosomes in higher organisms, such as fungi, plants, and animals, are mostly linear. The ends of linear chromosomes are protected by specialized structures called telomeres. A cell can contain, in addition to its own chromosome complement, extrachromosomal entities, or plasmids (Fig. 1-23). Most plasmids are double-stranded circles, 2000-100,000 bp (2-100 kilobase pairs) in size. Just as chromosomes do, plasmids carry genetic information. Due to their size and effect on the host cell, plas-mids can carry only a limited amount of information. The plasmid DNA duplex is compacted, or supercoiled. Breaking one strand of the plasmid duplex, or nicking, will relax the supercoil (Fig. 1-24), whereas breaking both strands will linearize the plasmid. Different physical states of the plasmid DNA can be resolved by distinct migration characteristics during gel electrophoresis.

Plasmids were discovered to be the source of resistant phenotypes in multidrug-resistant bacteria.47 The demon-

Circular plasmids (several thousand base pairs each)

Main circular chromosome (4 million base pairs)

Main circular chromosome (4 million base pairs)

Antibiotic-resistance genes

Genes necessary for DNA transfer

■ Figure 1-23 Plasmids are small extrachromosomal DNA duplexes that can carry genetic information.

Antibiotic-resistance genes

Genes necessary for DNA transfer

■ Figure 1-23 Plasmids are small extrachromosomal DNA duplexes that can carry genetic information.

station that multiple drug resistance in bacteria can be eliminated by treatment with acridine dyes48 was the first indication of the episomal (plasmid) nature of the resistance factor, similar to the F factor in conjugation. The plasmids, which carry genes for inactivation or circumvention of antibiotic action, were called resistance trans-

Locally denatured base pairs^

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